[imexhouse]

The reason I was getting that error was that in history_order.php I had 'pck_slip' instead of 'packing', so there was inconsistency in naming.
Make sure you're consistent. Also, if you copy the code from the forum, you'll end up with some spaces where there shouldn't be any.
Look at posts# 45-47

Oh, by the way, the original mod is balinor's and the batch feature chris.barber's.